From: dblack@...
Date: 2006-05-23T03:53:03+09:00
Subject: Re: initializing instance variables in a module

Hi --

On Tue, 23 May 2006, Jeff Rose wrote:

> Ryan Leavengood wrote:
>
>> On 5/22/06, Jeff Rose <rosejn@gmail.com> wrote:
>>> Have a module:
>>> 
>>> Module Foo
>>>    def add_stuff(key, stuff)
>>>      @foo_the_stuff[key] = stuff
>>>    end
>>> end
>>> 
>>> class Bar
>>>    include Foo
>>> end
>>> 
>>> Is there a standard or correct way to initialize @foo_the_stuff in this
>>> example?  Should the add_stuff method check whether the variable has
>>> been initialized every time, or should this be done by implementing a
>>> callback like append_features or included?  (That's how I'd want to do
>>> it, but it's not clear which or how...)
>>> 
>>> Thanks,
>>> Jeff
>> 
>> module Foo
>>   def add_stuff(key, stuff)
>>     (@foo_the_stuff ||= {})[key] = stuff
>>   end
>> end
>> 
>> If @foo_the_stuff is null, it will be initialized to an empty hash,
>> otherwise the existing value will be used. I consider this use of ||=
>> to be a Ruby idiom.
>> 
>> Ryan
>> 
> That's what I was afraid of.  In this example case it's no big deal, but in a 
> module where you are accessing a more complex variable many times it's a real 
> pain, let alone inefficient.  We are working on an event system, so the hash 
> is a hash of arrays or possibly a hash of hashes. Any of 4 methods could be 
> called, which would require an initialized value.  Having this in every 
> method is sorta lame...
>
>  handlers = (@event_handlers ||= Hash.new {|k,v| k[v] = []})[key]
>
> Of course we can pull it out to a method:
>
> def handlers(key)
>  @event_handlers ||= Hash.new {|k,v| k[v] = []})[key]
> end
>
> but just initializing on inclusion seems to be the most reasonable way, 
> doesn't it?

Not if it leads to code you strongly dislike :-)  It seems OK to wrap
it in a method, perhaps like this:

   def event_handlers
     @event_handlers ||= Hash.new {|k,v| k[v] = []}
   end

   def whatever ...
     handlers = event_handlers[key]
   end


David

-- 
David A. Black (dblack@wobblini.net)
* Ruby Power and Light, LLC (http://www.rubypowerandlight.com)
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   > http://www.manning.com/black